Optical fiber gratings are the key components in modern telecommunication systems. Optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. It is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical fiber gratings are important elements for selectively controlling specific wave-lengths of light within optical fiber communication systems. Gratings are used in controlling the paths or properties of light traveling within the fibers. Such gratings include Bragg gratings and long period gratings. Gratings typically comprise a body of material and a plurality of substantially equally spaced grating elements such as index perturbations, slits or grooves.
A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, including a plurality of perturbations in the index of refraction substantially equally spaced along the waveguide length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations times the effective refractive index, i.e. .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
Waveguide Bragg gratings are conveniently fabricated by doping a waveguide core with one or more dopants sensitive to ultraviolet light, e.g. germanium or phosphorous, and exposing the waveguide at spatially periodic intervals to a high intensity ultraviolet light source. The ultra-violet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
A difficulty with conventional Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects only light in a narrow bandwidth centered around .lambda.=2n.sub.eff .LAMBDA.. However in many applications, such as wavelength division multiplexing (WDM), it is desirable to have a reconfigurable grating whose wavelength response can be controllably altered.
One attempt to make a tunable waveguide Bragg grating uses a piezoelectric element to strain the grating. See Quetel et al., Paper No. WF6, 1996 Technical Digest Series,. Conf. on Optical Fiber Communication, San Jose. Calif., Feb. 25-Mar. 1, 1996, Vol. 2, p. 120. The difficulty with this approach is that the strain produced by piezoelectric actuation is relatively small, limiting the tuning range of the device. Moreover, it requires a continuous application of electrical power with relatively high voltage, e.g., approximately 100 volts.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two co-propagating modes with very low back reflections. A long-period grating typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance .LAMBDA.' which is large compared to the wavelength .lambda. of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing .LAMBDA.' which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.'.gtoreq.10.lambda.. Typically .LAMBDA.' is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5.LAMBDA.' to 4/5.LAMBDA.'. In some applications, such as chirped gratings, the spacing .LAMBDA.' can vary along the length of the grating.
Long-period fiber gratings selectively remove light at specific wavelengths by mode conversion. The spacing .LAMBDA.' of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a nonguided mode, thereby reducing in intensity a band of light centered about the peak wavelength .lambda..sub.p. Alternatively, the spacing .LAMBDA.' can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is substantially stripped off the fiber to provide a wavelength dependent loss. Such devices are particularly useful for equalizing amplifier gain at different wavelengths of an optical communications system.
A difficulty with conventional long-period gratings, however, is that their ability to dynamically equalize amplifier gain is limited, because they filter only a fixed wavelength acting as wavelength-dependent loss elements. Each long-period grating with a given periodicity (.LAMBDA.') selectively filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p. This wavelength is determined by .lambda..sub.p =(n.sub.g -n.sub.ng).about..LAMBDA.', where n.sub.g and n.sub.ng are the effective indices of the core and the cladding modes, respectively. The value of n.sub.g is dependent on the core and cladding refractive index while n.sub.ng is dependent on core, cladding and air indices.
In the future, multi-wavelength communication systems will require reconfiguration and reallocation of wavelengths among the various nodes of a network depending on user requirements, e.g., with programmable add/drop elements. This reconfiguration will impact upon the gain of the optical amplifier. As the number of channels passing through the amplifier changes, the amplifier will start showing deleterious peaks in its gain spectrum, requiring modification of the long-period grating used to flatten the amplifier. Modifying the long-period grating implies altering either the center wavelength of the transmission spectrum or the depth of the coupling.
Thus, there is a need for reconfigurable long-period gratings whose transmission spectra can be controlled as a function of the number of channels and power levels transmitted through an amplifier. It is desirable to have reconfigurable long-period gratings which, upon activation, can be made to dynamically filter other wavelengths (i.e., besides .lambda..sub.p). It is also desirable to be able to selectively filter a broad range of wavelengths. Further, reconfigurable long period gratings can be useful for suppressing amplifier spontaneous emission (ASE), and can also be used as tunable loss element for filtering out undesirable remnant signals from communication channel Add/Drop operations.
A related difficulty with conventional gratings is their temperature dependence. In the Bragg gratings, both n.sub.eff and .LAMBDA. are temperature dependent, with the net temperature dependence for a grating in silica-based fiber exemplarily being about +0.0115 nm/.degree. C. for .lambda.=1550 nm. The temperature-induced shift in the reflection wavelength typically is primarily due to the change in n.sub.eff with temperature. The thermal expansion-induced change in .LAMBDA. is responsible for only a small fraction of the net temperature dependence of a grating in a conventional SiO.sub.2 -based fiber. While such a temperature-induced wavelength shift can be avoided by operating the grating device in a constant temperature environment, it causes additional complications with a need to add an oven/refrigerator system. In addition, an accurate temperature-control and continuous use of power are called for.
U.S. Pat. No. 5,042,898 by W. W. Morey et al. discloses apparatus that can provide temperature compensation of a fiber Bragg grating. The apparatus comprises two juxtaposed compensating members that differ with respect to the coefficient of thermal expansion (CTE). Both members have a conventional positive CTE. The fiber is rigidly attached to each of the members, with the grating disposed between two attachment points. The apparatus is typically considerably longer than the grating, e.g. at least 40% longer than the grating device, thus making the temperature compensated package undesirably large. In addition, the temperature compensating packages can have a substantial variation of reflection wavelength from one package to another because of the variability in the grating periodicity as well as minute variations, during package assembly, in the degree of pre-stress applied to each grating or minute variations in the attachment locations.
In view of the foregoing, it can be seen that there is a need for compact, temperature-compensated optical gratings whose spacing can be latchably reconfigured.